Electric power cutters normally have a front and a rear handle each firmly attached to a drive unit including an electric motor. This results in that a lot of vibrations from the rotating tool and possibly the motor reach the handles and the operator. In many countries daily operating times are reduced by regulations according to the measured vibration level in the handle. It is known to attach one or both handles somewhat flexible with rubber bushings. This does reduce vibration levels to a certain degree, but has a limited effectiveness.
Electric tools, of which many are hand-held, must meet many requirements. They must be powerful and durable, yet remain easy to handle. A few of these tools are equipped with a high frequency brushless asynchronous or synchronous motor which exhibits a speed-torque characteristic that differs from ordinary AC motors tools. The speed of an asynchronous or synchronous motor at light loads is proportional to the frequency which also can be controlled to keep its speed relatively constant regardless of load conditions.
These motors are also known as brushless DC (BLDC) motors, and are generally reliable. Normally, they require a power supply device or a power converter, converting the mains frequency to the motor frequency, which often is higher than the mains frequency. The power converter is also called frequency converter.
The power supply device provides a suitable voltage and frequency to the tool, where the rotational speed of the motor is determined by the frequency of the supplied power. Such a power converter normally converts a mains input of, for example 3×400 V AC, 50 Hz, to a three-phase supply of 3×300 VAC, where the frequency is adjustable.
The power supply device or power converter is normally divided into two parts—an input rectifier and an output inverter. If instead the BLDC motor would be run from a DC source, e.g. a battery, no input rectifier would be needed.
The input rectifier converts the incoming AC input voltage to a DC output voltage, often called DC bus voltage.
The output inverter converts the DC bus voltage to three-phase voltages to the BLDC motor.
It is desirable for a tool with a BLDC motor to be able to operate over a broad range of AC mains voltages, e.g., from 3×180 VAC, 50/60 Hz, for the Japanese market and nominally 3×400 VAC, 50 Hz, for the European market. To achieve this, a power supply device according to the above is generally adopted to provide the same DC output voltage irrespective of the AC input. The common solution for this is to insert a converter assembly or voltage booster between the input rectifier and the output inverter. The power supply device will then consist of the following parts: Input rectifier, converter assembly or voltage booster and output inverter. An example of this is described in, for example, WO 2006009847.
The converter assembly or voltage booster converts (boosts) the incoming DC voltage to the output DC bus voltage, such that the output DC bus voltage normally is higher than, or the same as, the DC input voltage. Often the output DC bus voltage is a constant DC voltage, regardless of the input AC voltage.
However, there is still a need for an enhanced power supply device or power converter, that may provide sufficient output power with smaller and less expensive components than those disclosed in the prior art.